This invention relates to a hydrophilic urethane prepolymer composition useful, for example, as a sealant.
Urethane prepolymers used for sealing purposes must be uniformly dispersible in water and capable of forming nonshrinkable, resilient, and mechanically strong cured products having a cellular structure by the reaction with water. Hydrophilic urethane prepolymers have been used, utilizing these properties, for ground-accretion and wet wood-gluing purposes as well.
Japanese Patent Publication No. 25205/73 discloses a hydrophilic urethane prepolymer produced by reacting polyethylene glycol with an organic polyisocyanate. Japanese Patent Publication No. 19846/78 discloses a hydrophilic urethane prepolymer produced by reacting an organic polyisocyanate with a random or block copolymer of ethylene oxide and propylene oxide having an oxyethylene unit content greater than 50% by weight. Foam-forming hydrophilic urethane prepolymers are disclosed in Japanese Patent Kokai No. 47969/76, U.S. Pat. Nos. 3,894,131 and 3,985,688. They are mixtures of urethane prepolymers derived from polyethylene glycol having an MW of 850-1,000 and from polyethylene glycol having an MW of 1,250-1,550, the average MW of polyethylene glycol being 1,000-1,300.
The foam-forming hydrophilic prepolymers are advantageous in that, when injected into leaking sites in conjunction with water, the resulting foamed polyurethane mass is forced even into very fine spaces by the pressure of the carbon dioxide gas evolved by the reaction of isocyanate groups with water.
However, the prior art hydrophilic urethane prepolymers suffer from certain shortcomings. They tend to produce a foam having a relatively coarse cell texture. It is for this reason that some of previously cited patents teach the use of a surfactant to stabilize the foam but its efficacy has been proven to be not satisfactory. Furthermore, the foam produced from the prior art prepolymers are subject to shrinkage or compression because of their open cell structure. When this occurs, the polyurethane sealing layer peels from the concrete surfaces against which it seals so that it fails to perform the desired sealing function.
Accordingly, it is a principal object of this invention to provide a hydrophilic urethane prepolymer composition which is easily dispersible in water and capable of producing a non-shrinkable, closed cell type polyurethane foam having a fine cell texture and improved mechanical strength and elasticity.